Vatican representatives have faced tough questions from a United Nations committee on the Catholic Church's record of dealing with child sexual abuse by members of the clergy.

Members of the UN Committee against Torture (CAT) have probed Vatican officials on why more had not been done to prosecute abusive priests.

The head of the Vatican's delegation, Archbishop Silvano Tomasi, said the Holy See's powers did not extend beyond the Vatican, but added the church was taking action to address past wrongs.

Earlier this year, the UN Committee on the Rights of the Child accused the Vatican of fostering a climate of impunity with regard to child sexual abuse.

In a two-hour hearing in Geneva, CAT launched a barrage of questions to the Vatican delegation, asking about past policy decisions, the juridical distinction between the Holy See and Vatican City, and information on specific cases.

The Vatican, which will issue its formal answers on Tuesday, said the Church has been "doing its own house cleaning" for 10 years.

It says it is determined to protect children and that measures put in place have led to a decline in cases of sexual abuse of children by priests.

George Tugushi, a committee member from Georgia, said a recently formed international commission advising Pope Francis on how to deal with sexual abuse, was a very positive step but not enough.

"The commission may need help to ensure all cases are reported properly and begin to change the climate of impunity, but it cannot be considered in our opinion as a substitute for a functioning investigation system," he told the Vatican delegation.

Another committee member, Satyabhoosun Gupt Domah of Mauritius, asked if the Holy See was taking steps to eliminate the "chemistry that creates the conditions" for sexual abuse of children by priests.

The Holy See's position is its adherence to the UN Convention against Torture applies only to the territory of Vatican City.

Archbishop Tomasi said while the Holy See can be a moral force, the "agent of justice" for crimes committed by Catholics was the local state where the crime was committed.

"It should be stressed, particularly in light of much confusion, that the Holy See has no jurisdiction ... over every member of the Catholic Church," he said in opening remarks.

Claims Vatican is splitting hairs

Barbara Blaine of the Survivors Network of those Abused by Priests (SNAP) accused the Vatican of ducking responsibility.

"They are splitting hairs when they should be embracing the victims and stopping the sexual violence," she said.

The committee's chief rapporteur, Felice Gaer of the United States, told the Vatican delegation that its position "seems to reflect an intention for a significant portion of the actions and omissions of Holy See officials be excluded from consideration by this committee, and this troubles us".

Mr Gaer and Mr Tugushi presented dozens of questions to the delegation, asking them to respond to reports presented to the committee by non-government organisations.

"We have received numerous allegations of intimidation of witnesses and shifting of finances to avoid payment (of compensation)," Ms Gaer said.

Archbishop Tomasi told news agency Reuters that the reduction in the number of cases of abuse showed that "effective" action taken by the Church was working.

"I think that in this kind of situation there will never be enough done. The damage has been done, reparation has to continue," he said.

Church groups defend "strong" reforms and protocols

The Church in the United States had invested $US2.5 billion in compensation for the victims. Archbishop Tomasi said "most of the abuser priests that we know of have been defrocked".

Church groups defended the Catholic Church's efforts to stem abuse and criticised committee members who said the Church's opposition to abortion had harmed women.

"Attacking the Church's moral and religious beliefs violates the religious liberty of the Church, a human right which the United Nations affirms," said Ashley McGuire of Catholic Voices.

"Over the last decade, the Church has put into place reforms and protocols so strong that they are now being modelled by other institutions, like public schools," she said.

Last February, a UN committee on the Rights of the Child accused the Vatican of systematically turning a blind eye to decades of abuse and attempting to cover up sex crimes.